helper/binarybuffer: fix clang static analyzer warnings

Writing bits to an uninitialized buffer generated false warnings.
Zero buffers before setting them by buf_set_u32|64()
(do it only if bit-by-bit copy loop is used,
zeroed buffer is not necessary if a fast path write is used)

Change-Id: I2f7f8ddb45b0cbd08d3e249534fc51f4b5cc6694
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5383
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>

src/target/openrisc/or1k_tap_vjtag.c

@@ -149,7 +149,7 @@ static int or1k_tap_vjtag_init(struct or1k_jtag *jtag_info)
 	 * into the USER1 DR is sufficient to cover the most conservative case for m and n.
 	 */
 
-	uint8_t t[4];
+	uint8_t t[4] = { 0 };
 	struct scan_field field;
 	struct jtag_tap *tap = jtag_info->tap;